Days Inventory Outstanding (DIO) measures how long, on average, a unit sits in the warehouse before a customer buys it.
365 annual, 90 quarterly. Inventory valued at cost so numerator and denominator are like-for-like.
The period is 365 for an annual read and 90 for a quarterly one. Inventory is valued at cost rather than retail so the numerator and the COGS denominator are stated in the same dollars — a like-for-like ratio, not revenue against cost.
The “average inventory” choice matters more than the formula suggests. The textbook convention is (opening inventory + closing inventory) / 2, which works fine for brands whose stock levels are roughly flat through the period. For brands with seasonal spikes — Q4-heavy apparel, gifting categories — a single year-end endpoint reads off the trough after the season has run, and the two-point average understates how much inventory the business actually carried. A monthly rolling average across the period smooths the read in those cases.
DIO sits inside the cash conversion cycle: CCC = DIO + DSO − DPO. For a pure-DTC brand on Shopify Payments and similar processors, DSO collapses toward zero because card-on-checkout settles within about three business days in the US (regional variation exists). That leaves DIO as the component most directly under operator control — receivables aren’t a knob a DTC team turns, but the buy plan is.
DIO and inventory turnover encode the same information in two presentations: turnover = days in period / DIO. Turnover reads as a multiple (“4× per year”); DIO reads in days and composes cleanly into CCC by addition. Finance usually anchors on DIO because it speaks the same units as the working-capital cycle; ops teams often prefer turnover because “we’re turning four times a year” lands faster on the floor. Same number, two dialects.
What pushes the number up: deeper safety stock, longer supplier lead times, pre-buys for seasonal launches, and slow-moving tail SKUs that sit while the hero items churn. What pulls it down: a tighter buy plan, faster sell-through, and ABC discipline on the long tail. The tradeoff is real — running leaner protects working capital and reduces carrying cost (a direct lift to gross margin through lower obsolescence and storage), but cuts into service level when demand spikes. A brand running $200K/month in inventory at 90-day DIO has roughly three months of COGS tied up in stock; shaving 15 days frees on the order of a half-month of COGS, available to redeploy.